'69 C10 327 SBC Off Idle Stumble / Misfire Help - I feel like I've tried EVERYTHING

[SRT8-X]

Originally Posted by geezer#99

Timing can do that.
Too much fuel pressure can do that.
A valve hung open can do that.
Lots of possibilities.

You gotta start over.
Rerun your valves.
Optimize your timing.
Verify your fuel pressure at all rpm’s.
Adjust your carb.
See what it does then.

If your timing light is wacky like you mentioned just replace #1 wire with a normal stock type wire.

Thanks again! I'll get back on this after checking it out, looking at the valves and compression.

[paul blair]

Hi Keith, Do a compression check and see if that tells you some thing! I had a broken valve spring on one cylinder and chased the ignition and fuel systems and did not find the problem until a compression test and a visual on the valve springs. At least you will know if you have a tight engine!

[SRT8-X]

Originally Posted by paul blair

Hi Keith, Do a compression check and see if that tells you some thing! I had a broken valve spring on one cylinder and chased the ignition and fuel systems and did not find the problem until a compression test and a visual on the valve springs. At least you will know if you have a tight engine!

Thanks Paul! I'm headed this route after a few suggestions. I'm going to take a look at the valve lash and springs too. I appreciate it.

[geezer#99]

Check for uniform rocker movement.
I’ve used tall pieces of cardboard as oil deflectors when running the valves.
When you set them just start with #1 and back off each nut until they click and then tighten just enough to be quiet. Do all that way. Then shut the motor off and adjust all to 1/2 turn.
Sometimes if you’re adjusting the preload too quick you can cause the motor to stumble and die.
Half a turn gets you close depending on whose lifters are in it. Factory setting is likely 1 turn but not all lifters are created the same. Some design differences from cam company to cam company.
Then do a compression check.

[68 P.O.S.]

Your HEI specs sound fine, just have to verify it’s working properly. After your rewiring job, it should be getting the full 12 volts, so just verify that to be sure. Is the vacuum advance adjustable? Normal HEI spark plug gap is .045, but if DUI suggests .050 for their unit, keep it there.

Since you’re a little confused on timing, here’s the skinny. Initial timing is just the engine by itself without any mechanical or vacuum advance added in. Rotating the distributor adjusts it and gets the engine in the ballpark to ensure its working well. Basically a coarse adjustment. Total timing is the engine’s initial timing, plus however many degrees of mechanical advance the distributor has. This fine tunes the initial and mechanical advances together. To adjust it, the engine is rev’d to high rpm (3-4K + rpm, depending on the spring strength) until it stops advancing, which is when full mechanical advance is reached. Personally I just take the springs out (don’t lose them!) and rev it between 1500-2000 rpm to set it because, #1, I’m not too fond of the engine screaming at 4K rpm while my head is under the hood, and #2, full mechanical advance is achieved at a much lower rpm without the springs holding it back. Using your engine and 36 degrees total timing an example….24 degrees mechanical advance, plus 12 degrees initial timing, equals 36 degrees total timing. 24+12=36. That’s your total timing. Then there’s the vacuum advance. The vac advance is used to add an extra amount of timing to increase the engine’s efficiency during low load situations; idle and cruise. Rich mixtures burn quickly, lean mixtures do not. Idle and cruise are lean operating conditions and need the extra timing from the vac advance to burn the lean mixture efficiently. So to achieve this, your vac advance must be plugged in to manifold vacuum. Timed, or ported vacuum, is used for emissions equipment your engine may be required to run, such as EGR valves, smog stuff, etc. Our trucks aren’t encumbered with that, so we can use the vac advance to its full advantage. Adding too much timing will cause detonation, so 10-15 degrees vac advance is plenty. Once hooked up to manifold vac, the extra timing will increase the idle, so you can lower it back down to 750 rpm and you should be able to adjust your idle mixture screws as outlined in the manual. Here’s a timing article written by a GM engineer…more than you want to know I bet, ha, but invaluable info: http://www.camaros.org/pdf/timing101.pdf

Valve adjustment is a great idea as well. Use Comp Cams’ EOIC method for adjusting them dead on.

[notsolo]

Just changed the gap in my spark plugs, old ac plugs I was told .045-.050.....went back to .035...runs alot better...Just my 2 cents I know nothing about high voltage coil...etc.

[AussieinNC]

As I mentioned I suspected the distributor and or module is not working correctly...

Your mentioning dropping the plug gap down to .035 and it runs better, confirms a lack of high voltage for the spark system...

To check the system, drop in a standard HEI distributor setup and retry your test drive...dont be surprised if it fixes the off idle stumble and misfire...

[SRT8-X]

Originally Posted by AussieinNC

As I mentioned I suspected the distributor and or module is not working correctly...

Your mentioning dropping the plug gap down to .035 and it runs better, confirms a lack of high voltage for the spark system...

To check the system, drop in a standard HEI distributor setup and retry your test drive...dont be surprised if it fixes the off idle stumble and misfire...

Thanks AussieinNC, however that was another individual posting about the gaps. I put out a lot of information so if it was missed, I had an original OEM HEI set-up on this engine when setting it up initially. It had the same issue then and after just replacing the cap, rotor and coil. Then I replaced the entire distributor and the module, still the same.

Same story with the plugs, originally gapped at 0.035 and A86's then went to AP86's gapped at 0.050. Same result. I tried Performance Distributors on the phone all day today and got a busy signal so I sent a message. They do offer to have customers send in the units for testing and repair and if they don't find anything, they charge a $25 service fee so that's a viable option if I can't rule ignition out.

[AussieinNC]

Thx for the update...

This may sound silly,,, but there is a good ground strap in place between the engine block and the cab of the truck?

I have witnessed a simple missing ground strap drive guys insane for weeks....

[SRT8-X]

Originally Posted by 68 P.O.S.

Your HEI specs sound fine, just have to verify it’s working properly. After your rewiring job, it should be getting the full 12 volts, so just verify that to be sure. Is the vacuum advance adjustable? Normal HEI spark plug gap is .045, but if DUI suggests .050 for their unit, keep it there.

Since you’re a little confused on timing, here’s the skinny. Initial timing is just the engine by itself without any mechanical or vacuum advance added in. Rotating the distributor adjusts it and gets the engine in the ballpark to ensure its working well. Basically a coarse adjustment. Total timing is the engine’s initial timing, plus however many degrees of mechanical advance the distributor has. This fine tunes the initial and mechanical advances together. To adjust it, the engine is rev’d to high rpm (3-4K + rpm, depending on the spring strength) until it stops advancing, which is when full mechanical advance is reached. Personally I just take the springs out (don’t lose them!) and rev it between 1500-2000 rpm to set it because, #1, I’m not too fond of the engine screaming at 4K rpm while my head is under the hood, and #2, full mechanical advance is achieved at a much lower rpm without the springs holding it back. Using your engine and 36 degrees total timing an example….24 degrees mechanical advance, plus 12 degrees initial timing, equals 36 degrees total timing. 24+12=36. That’s your total timing. Then there’s the vacuum advance. The vac advance is used to add an extra amount of timing to increase the engine’s efficiency during low load situations; idle and cruise. Rich mixtures burn quickly, lean mixtures do not. Idle and cruise are lean operating conditions and need the extra timing from the vac advance to burn the lean mixture efficiently. So to achieve this, your vac advance must be plugged in to manifold vacuum. Timed, or ported vacuum, is used for emissions equipment your engine may be required to run, such as EGR valves, smog stuff, etc. Our trucks aren’t encumbered with that, so we can use the vac advance to its full advantage. Adding too much timing will cause detonation, so 10-15 degrees vac advance is plenty. Once hooked up to manifold vac, the extra timing will increase the idle, so you can lower it back down to 750 rpm and you should be able to adjust your idle mixture screws as outlined in the manual. Here’s a timing article written by a GM engineer…more than you want to know I bet, ha, but invaluable info: http://www.camaros.org/pdf/timing101.pdf

Valve adjustment is a great idea as well. Use Comp Cams’ EOIC method for adjusting them dead on.

Thanks 68 POS, I appreciate the informative response. I'm good with the timing now, thanks for the refresher, I just wasn't sure regarding the original calibration they put on the distributor.

[geezer#99]

Sounds like a float problem.
When you check the float level also check the float drop. It has to be set right too.

[Fbks_Cruiser]

Quote: “I lowered the idle to 650 RPM or so, set initial to 12° (it was confirmed only the 4° off from my initial 16° setting so timing was accurate anyway) and 36° at 3000-3200 RPM.

I ran the engine at various RPM in the garage while working on this and getting to operating temps. This was likely 25-30 minutes as I was doing a few other things, looking at vacuum (dropped about 8-10" due to idle speed)” Quote

Your initial post said you had about 20” of vacuum at idle. Does this mean you now only have 10” vacuum at idle? That is way low. Barring a vacuum leak somewhere, such low vacuum indicates possible valve issues as discussed in other posts

[HO455]

A couple of items that may help. It sounds like backing the timing off helped. Have you verified that your timing marks for TDC are correct?

Misfires while turning does suggest a float level problem. When you check the float level make sure that the floats are not fuel soaked or if brass that they have no fuel inside of them. Also check that they move freely when holding the air horn level and at angles to mimic the forces felt when turning.

Your plug wires could have high internal resistance that may be changing as you move the wires. Here is what I use to test plug wires and spark power.
https://www.amazon.com/Lisle-20700-C.../dp/B00IXMA1RY

Have you tried wiring the choke open to take it out of the equation?

It is quite possible that you have more than one problem contributing to your symptoms.

Here's a link to a valve adjustment procedure.

https://www.crankshaftcoalition.com/...raulic_lifters

[SRT8-X]

Originally Posted by HO455

A couple of items that may help. It sounds like backing the timing off helped. Have you verified that your timing marks for TDC are correct?

Misfires while turning does suggest a float level problem. When you check the float level make sure that the floats are not fuel soaked or if brass that they have no fuel inside of them. Also check that they move freely when holding the air horn level and at angles to mimic the forces felt when turning.

Your plug wires could have high internal resistance that may be changing as you move the wires. Here is what I use to test plug wires and spark power.
https://www.amazon.com/Lisle-20700-C.../dp/B00IXMA1RY

Have you tried wiring the choke open to take it out of the equation?

It is quite possible that you have more than one problem contributing to your symptoms.

Here's a link to a valve adjustment procedure.

https://www.crankshaftcoalition.com/...raulic_lifters

HO455, thanks for the updated response. Yes, I've verified TDC and the marks several times. My new mark is about 4° behind that of the original mark on the balancer.

Noted on the floats, I appreciate this information, I will check this as well.

Regarding the choke, I have not wired it open but have run several settings. I may give this a try as well but the choke seems to function properly and visibly open when it's supposed to be. Another note, the same situation was apparent with the old Carter AFB as well, it had the same or similar problems with the old carb and intake set-up.

Thanks for the link on valve adjustment, I'll plan to tackle this on the upcoming weekend and visually inspect the springs as well.

[SRT8-X]

Originally Posted by Fbks_Cruiser

Quote: “I lowered the idle to 650 RPM or so, set initial to 12° (it was confirmed only the 4° off from my initial 16° setting so timing was accurate anyway) and 36° at 3000-3200 RPM.

I ran the engine at various RPM in the garage while working on this and getting to operating temps. This was likely 25-30 minutes as I was doing a few other things, looking at vacuum (dropped about 8-10" due to idle speed)” Quote

Your initial post said you had about 20” of vacuum at idle. Does this mean you now only have 10” vacuum at idle? That is way low. Barring a vacuum leak somewhere, such low vacuum indicates possible valve issues as discussed in other posts

Sorry for the mistake, I meant to write 8-10%. The vacuum at the low idle was around 18-18.5" Hg. It still had a minor drop / fluctuation at idle at times and was steady at times as well during this tuning.

[SRT8-X]

Originally Posted by geezer#99

Sounds like a float problem.
When you check the float level also check the float drop. It has to be set right too.

Thanks again geezer, I will be checking the float and float drop this week.

[Ironangel]

Run your valves again to verify you dont have one or more hanging open. Use the EOIC method, never mind cutting a valve cover. Secondly, maybe I missed it but what cam & lifters are you running? Thirdly, find a proven functional carb off another small block with comparable cam and heads just to verify a carb problem. Surely a friend or board member, perhaps a local speed shop or drag racer has a carb laying around to loan for a couple of test hits?...Lastly, read up on mods to the accelerator pumps on those AFB carbs. With those heads and a comparable semi race cam a stock 600 cfm AFB squirter aint going to cut it. I'm thinking either more volume of the squirt, and/or a longer duration of the squirt will make your 327 snap to attention...I'd loan you a proven Cliff Ruggles Quadrajet if you were closer and running a spread bore intake...Good luck, your well on your way to becoming an expert AFB guy!

[SRT8-X]

Originally Posted by Ironangel

Run your valves again to verify you dont have one or more hanging open. Use the EOIC method, never mind cutting a valve cover. Secondly, maybe I missed it but what cam & lifters are you running? Thirdly, find a proven functional carb off another small block with comparable cam and heads just to verify a carb problem. Surely a friend or board member, perhaps a local speed shop or drag racer has a carb laying around to loan for a couple of test hits?...Lastly, read up on mods to the accelerator pumps on those AFB carbs. With those heads and a comparable semi race cam a stock 600 cfm AFB squirter aint going to cut it. I'm thinking either more volume of the squirt, and/or a longer duration of the squirt will make your 327 snap to attention...I'd loan you a proven Cliff Ruggles Quadrajet if you were closer and running a spread bore intake...Good luck, your well on your way to becoming an expert AFB guy!

Thanks Ironangel! I appreciate the advice. I was thinking old school with the valve adjustment when I suggested that but have since been turned on to the EOIC method and watched a few good videos on this. I just remember seeing my dad do this running a few decades ago and haven't needed to adjust an SBC valves until now.

Regarding the cam, it's mild at best, no lope or choppy idle but I cannot be certain what the specs are on the cam and lifters. I'd say its similar to what may have been in this 327 stock and I doubt it was one of the HP 'Vette units. My knowledge is that it was from a '65 Nova but even this could have been 275HP+.

Noting again that I had the previous Carter AFB 600 CFM with similar issues, this makes me believe it's not the carb since my dad was a carb guru and rebuilt AFB, Holley and Q-Jet's for a hobby. I'll definitely take a look at a borrowed carb if the float adjustment doesn't do the trick as well as accelerator pump alternates.

Thanks again for the comments and encouragement!

[SRT8-X]

I'm posting the following picture of the lifter valley just to determine if anyone sees any lifters which look out of position or sequence. I know I need to verify / adjust the valves but figured one of you may be able to visually see something wrong here. I don't know what position the engine is at this point, it was when I was installing the new intake and carb. Just figured it may help identify an issue with the valves if present.

[geezer#99]

Remember that when you do the valves again, if you do it cold and use the eoic method, wiggle the slack out of the rocker. Twisting the rod is harder to get zero lash right on.

[68 P.O.S.]

Another thing to look at is the accelerator pump assembly. I once had the rubber plunger un-seat itself from the pump rod and was floating around freely. It sort of worked still, but it obviously wasn't doing it's job right. I pulled it out and re-seated it, but it did it again a couple days later. So I bought the following accelerator pump assembly and that did the trick: https://www.edelbrock.com/carburetor...mbly-1470.html. The blue plunger at the end of the assembly in the pic is what I'm talking about.

[Gromit]

Noticed the crankcase breather hose is plugged in the photo - maybe that is just because you have it apart so just a reminder to have some means of venting the crankcase.. no venting could definitely cause tuning issues.

My 1970 350/350 truck has the stumble issue but not often enough to be a problem - I believe on mine it is the stock torque converter not very well matched to the torque curve of the cam. So the posters who have asked about your cam I would second their questions - your cam specs are important to have documented in solving your issue.

I can affect the stumble by adjusting my vacuum secondary rate but that adjustment seems to change with the weather..

[SRT8-X]

Originally Posted by Gromit

Noticed the crankcase breather hose is plugged in the photo - maybe that is just because you have it apart so just a reminder to have some means of venting the crankcase.. no venting could definitely cause tuning issues.

My 1970 350/350 truck has the stumble issue but not often enough to be a problem - I believe on mine it is the stock torque converter not very well matched to the torque curve of the cam. So the posters who have asked about your cam I would second their questions - your cam specs are important to have documented in solving your issue.

I can affect the stumble by adjusting my vacuum secondary rate but that adjustment seems to change with the weather..

Gromit, that's correct, I have a breather with integral PCV valve in the passenger valve cover typically. It's plugged for the intake replacement only.

As noted, the cam is likely a stock or extremely mild grind, it has a smooth idle and no aggressiveness so I'd suspect a low end power range. I will probably be replacing the cam and valvetrain, possible aluminum free flowing heads too, after everything is ruled out. Thanks for your comment.

[geezer#99]

Have you checked the valve lash yet?

That’s the first thing you should do.

[SRT8-X]

Originally Posted by geezer#99

Have you checked the valve lash yet?

That’s the first thing you should do.

Geezer, I have not had a chance to work on the '69 since Monday. Will be resuming this evening and Saturday. I'm taking all suggestions into account and will verify everything but I wanted to look at the carb to verify since this is recommended on new units anyways. I'll be pulling the covers to check the valve lash regardless. Thanks.